Solid-state image sensors have found widespread use in digital camera systems. Solid-state image sensors use an array of picture elements (pixels), typically arranged in rows and columns, to convert electromagnetic (EM) energy (e.g., infrared, visible light, ultraviolet light, x-rays, etc) into a charge that can be detected and processed to generate a digital image. An array with only one line (one column or one row) of pixels is known as a linear array, while an array with multiple lines (multiple rows and columns) is known as an area array. While many different semiconductor processing technologies may be used to produce solid-state image sensors (e.g., NMOS, PMOS and BiCMOS), the two principle technologies used for solid-state image sensors are CMOS (complementary metal-oxide semiconductor) technology and CCD (charge-coupled device) technology.
CCD image sensors are based on charge generation (in response to EM energy exposure) within an array of pixels, and charge transfer across the array in a type of “bucket brigade” operation. Analog voltages corresponding to the charge generated at each pixel are read from the CCD sensor by applying clocking signals to transfer the charge in each row of pixels to the succeeding row (parallel transfer) and eventually to a serial register from which charges are clocked and transferred to a detector and amplifier.
CMOS image sensors also generate pixel charges in response to EM energy exposure. In contrast to CCD image sensors, however, the charges are not transferred from pixel to pixel. Rather, all of the pixels in a column of a CMOS sensor share a column bus and the signals are read out sequentially by the operation of switches (typically MOS field-effect transistors) within each pixel to achieve a column parallel, row serial readout order. Conventionally, both CCD arrays and CMOS arrays generate an n pixel by m pixel image from a sensor array having n rows and m columns.
Linear array (i.e., one line) CCD and CMOS sensors may be used to generate two dimensional images by repeatedly exposing and reading a single row sensor while moving the sensor in a direction orthogonal to the long dimension of the array. In this so-called “push broom” technique, the resulting image has a maximum width in pixels equal to the number of pixels in the linear array, and a theoretically unlimited length corresponding to the number of samples. This is the principle behind common scanning devices such as facsimile machines and document copiers.
In “push broom” imagers, the long dimension of a one-line imaging array (e.g., 1×512 pixels) is positioned perpendicular to the direction of the motion between the imaging platform and the imaged scene. The direction of the motion is known as the “along-track” direction and the direction perpendicular to the direction of motion is known as the “cross-track” direction. In the push broom method, the array is used to divide the scene into pixels in the cross-track direction and the imaging array is sampled in time to capture the scene in the along-track direction as the array moves. Ideally, the time sampling is synchronized with the velocity of the imaging array so that the image of the scene (in the focal plane of the imaging array) moves by the length of a pixel in the along-track direction in the time it takes to expose the pixel, extract image information from the pixel and condition the pixel for the next exposure. If this timing can be accomplished, then a two-dimensional image of the scene can be formed by processing a continuous sequence of one-line “slices.” One significant problem with the push broom technique is that the charge integration time (exposure time) per slice is limited by the velocity of the imaging platform. Limited integration time translates to low signal levels (low charge generation) and a commensurately low signal-to-noise ratio (SNR) in the acquired image (every image sensor has noise sources such as shot noise and thermal noise). TDI was developed to increase the SNR of moving image sensors.
TDI image sensors (TDI imagers) use an area array image sensor to capture images from an imaging platform that is moving with a constant velocity relative to an imaged object or scene. One common application of TDI is terrestrial imaging from satellite or aircraft borne platforms where the imaged object or scene is known as the “ground scene.” Other applications include the scanning of objects as they move on a conveyor belt, for example (i.e., machine vision). For clarity of explanation, the following background information uses terminology that applies to terrestrial imaging applications, although the concepts apply equally well to all TDI applications.
A TDI imager can be viewed (conceptually) as a stack of linear arrays (i.e., an area array), where every linear array moves over the same ground scene points (ground pixels), separated by the time required for the sensor to move one line with respect to the ground scene (the line time, TL). Any particular ground scene point is sampled by a column of individual pixels in the array at multiples of the line time. If those individual pixel samples can be added or accumulated, then the SNR of the resulting image can be increased by a factor equal to the square root of n, where n is the number of pixels in a column of the TDI imager.
Conventionally, only CCD technology has been used for TDI applications because CCDs naturally operate by transferring charge from pixel to pixel across the focal plane of the sensor, allowing the charges to be to be integrated (added) from pixel to pixel as the sensor moves over a ground pixel in the imaged scene. However, CCD technology is relatively expensive and CCD imaging devices consume much more power (100 to 1000 times more) than comparably sized CMOS devices.
A CMOS TDI sensor having active pixels with snapshot capability has been disclosed (see Pain et al., “CMOS Image Sensors Capable of Time-Delayed Integration,” NASA Tech Brief Vol. 24, No. 4, pp. i, 1a-8a). Snapshot capability refers to a CMOS pixel structure that uses four or more transistors, including a transfer gate, to isolate charge from one charge integration period from the charge stored in the pixel from a previous charge integration period. Snapshot operation allows the charge integration and readout operations in pixel to be managed independently. However, while a pixel structure with snapshot capability can be used to implement a CMOS TDI sensor, it has disadvantages. In particular, the fill factor and quantum efficiency of a 4T to 6T pixel is less than a three transistor (3T) pixel structure that does not have a snapshot capability.